Constant temperature oven



y 1935- c. WATSON 2,000,266

CONSTANT TEMPERATURE OVEN Filed April l8, 1932 EWWMWWW/JZZZ INVENTOR, CHARLES L. WATSON.

BY M/Z/M A TTORNE Y Patented May 7, 1935 t s S PATENT OFFICE 3 a "cons-ram TEMPERATURE oven Charles L. Watson, San Francisco, Calif. Application A -u 1a ,'19a2,senai No. 606,062

, 5 claims. (or. 219-19) My invention relates to ovens usedto keep suited for airplane use. Broadly consideredit an article at a substantially constant temperacomprises a flat metal plate acting as a thermal ture over long periods of time, and more partransmission line having, its periphery exposed ticularly to an oven used to keep apiezo-electric to the surrounding air, a casing containing a 5 crystal at a substantially constant temperature heater and a thermostat, on one side of the 5 while being employed to stabilize the frequency metal plate, and a casing, containing the object of an oscillating circuit. a I to be .maintained at a constant temperature Among the objects of my invention are: to on the other side. The thermal transmission providean oven having a substantially constant line receives heat nearits center and radiates temperature node; to provide an oven having this heat into the ambient air through the ex- 10 a constant temperature node without resorting posed periphery. to the use of large amounts'of heat insulating vA preferred form of my invention is shown material; to provide a constant temperature in the drawing. It comprises a thermal transoven which ishighly'compact; to provide a com mission line [preferably a flat plate of copper or stant temperature oven which is light in weight; other heatconducting material. This plate pref- 15 to provide an oven which may conveniently be erablyhas its lower surface blackened, and its used in aircraft; and to provide a simple and upper surface, highly polished, and platedwith economical oven wherein a piezo-electric crystal chromium or, silver, for purposes hereinafter. control element of a high frequency alternating described. Mounted on the upper surface of the 30 current circuitmay be kept at a substantially plate isa crystal casing 2, and on the lower constant temperature-over longperiods of time. surface is a heater'casing 4. These casings are Other objects of my' invention will :be apparpreferably of, moulded Bakelite .or other macnt or will be specifically pointed out in the terial of a'semi-heat insulating character. description forming apartof this specification, The heater casing has. contained therein two UNITED. STATE but I donot limit myself to the embodiment of heater coils .5 of resistance wire, stretched be- 25- my invention herein described, as various forms tween studs 6 and contact pins I which are may he adopted within the scope of the claims. secured to the bottom of the easing. Between Referring to the drawing: a these coils, and mounted on studsjB are two Figure 1 is a top view of a preferred form thermostat arms 9, of bimetallicmaterial, carof oven. g r rying at their extremities electrical contact 30 Figure 2 is a cross sectionalvie'w taken points l0. These are normally held in contact through lines 22 in Figurel. I by adjustment screws II. When current is ap- Figure 3 is an inside surface view of the lower plied 'to the circuit, heat from coils 5 causes the part of Figure 2 as designated at line 3-3. arms to separate, breaks the circuit, and allows 5 Figure 4 is a sectional view taken through the coils to cool. Clrcuitis thereupon reestabline 4-4 in Figure 1. I I lished' and'the cycle is'repeated, the ofi and Figure5 is an inside surface view of the upper on periods being conveniently adjusted by portion of'Figure 2 as designated at line 5-5. screws ll. During the heating period, the cen- It is well known in the art that pieao-electric tral portion of plate I is also exposed to heat 40 crystals are used to control the frequency of from coils 5. The blackening of the'lower por- 40 oscillators, and it is also well known that tion of plate I as described above, prevents heat changes in temperature of the crystals will reflection and promotes absorption by the plate. change their response frequency. It has hereto The thermostat arms are exposed to the temfore been necessary to enclose such crystals in perature of the plate as well as to that of the 4,5 ovens, surrounded by multiple heat insulating coils; Also contained in this casing is thecon- 4-5 coatings, sometimes several feet in thickness, tact post l3 which makes connection to plate I in an endeavor to shield the crystal and assoby means'ofspring I4. i ciated elements from all influence of ambient Crystal casing 2 has two compartments l2 air temperatures. Such ovens are obviously exmoulded therein. Each compartment contains a pensive, bulky, and totally unsuited for use in piezo-electric crystal 15, one face resting di- 50 aircraft. Many of them take several hours to rectly on the polished surface of plate I, which reach equilibrium. thereby becomes one electrode in the crystal cir- My invention makes direct use of contact with cuit. This surface as described above should ambient air temperatures, has no thick insulatbe highly polished and may be plated with ing coverings, is small, compact and ideally chromium or silver in order to preserve the sur- 55 face. Many crystals refuse to oscillate if the electrode surfaces are rough. On the opposing face of the crystals is placed an electrode l6, also polished and plated at the plane of contact, held with light pressure by springs H, which in turn are attached to connecting posts I9. The piezo-electric crystals may be identical or responsive to different frequencies.

Cases 2 and 4 together with their respective contents, are held against the opposite faces of plate I by bolts 20 and screws 2!, thereby forming the complete oven assembly. It is important that the periphery of plate I be left exposed to the surrounding air.

Convenient dimensions of the complete oven in its preferred form are as follows:

Inches Length 6 Width 3 Thickness 2 Thickness of plate When connected to a source of current the thermostat operates to maintain the temperature within the casing 4 constant between the predetermined limits for which it is set. Heat is lost from this casing through the transmission line, and also through the wall of the casing. The heat is supplied to the plate primarily by radiation and the heating elements are closely adjacent the transmission line, while the thermostat is more closely adjacent the outer wall of the casing. It follows that convection is low, and that the maximum temperature of the plate is maintained higher with low ambient temperatures than with high, giving a steeper temperature gradient along the line.

There will, however, be a temperature node in the line, whose temperature remains substantially constant, when the temperature of the periphery rises or falls and that closest to the heating coils falls or rises. Ambient air temperatures may vary widely without substantially changing the temperature of this node. I prefer to place the piezo-electric crystals in the locality of this node. Equilibrium is established in from 10 to minutes and thereafter crystals or other objects placed on the constant temperature node will be maintained at a substantially constant temperature as long as heating current is supplied.

Casings 2 and 4 being semi-insulators of heat merely act to prevent drafts and sudden changes of temperature from affecting the apparatus directly.

Oscillating circuits controlled by crystals kept in the oven of my invention have maintained a frequency with a variation of less than 10 cycles per million over long periods of time. This variation is less than that required by present government regulations.

I claim:

1. In combination, a plate of heat conducting material, a block of heat insulating material, having a recess therein, disposed on each side of said plate to provide a pair of opposed fluid tight chambers separated by said plate, a thermostat and a heater controlled thereby enclosed in one of said chambers, the other of said chambers adapted to contain an object to be heated, said plate having areas thereof exposed to the air in said chambers, an area exposed to ambient air, and areas intermediate said exposed areas shielded from the air in said chambers and from ambient air.

2. In combination, a plate of heat conducting material, a block of heat insulating material, having a recess therein, disposed on each side of said plate to provide a pair of opposed fluid tight chambers separated by said plate, a thermostat and a heater controlled thereby enclosed in one of said chambers, the other of said chambers adapted to contain an object to be heated, said plate having areas thereof exposed to the air in said chambers, the periphery thereof exposed to ambient air, and areas adjacent said periphery shielded by portions of said blocks from the air in said chambers and from ambient air.

3. In combination, a plate of heat conducting material having a portion thereof exposed to ambient air, a block of heat insulating material, having a recess therein, disposed on each side of said plate to provide a pair of opposed fluid tight chambers separated by said plate, a thermostat and a heater controlled thereby enclosed in one of said chambers, the other of said chambers adapted to contain an object to be heated, said blocks contacting areas of said plate adjacent the periphery thereof to establish a heat transmission line through said plate between portions thereof adjacent said chambers and the periphery thereof, said transmission line being shielded from the air in the chambers and from ambient air.

4. In combination, a pair of blocks of heat insulating material having recesses therein facing each other, a plate of heat conducting material between said blocks, opposed surfaces of said plate forming a closure for said recesses to provide a pair of oppositely disposed fluid tight chambers, a thermostat and a heater controlled thereby enclosed in one of said chambers, an object to be heated enclosed in the other of said chambers, and said plate having a relatively small area thereof exposed to ambient air to permit heat drainage from said chambers.

5. In combination, a pair of blocks of heat insulating material having recesses therein facing each other, a plate of heat conducting material between said blocks, opposed surfaces of said plate forming a closure for said recesses to provide a pair of oppositely disposed fluid tight chambers, a thermostat and a heater controlled thereby enclosed in one of said chambers, an object to be heated enclosed in the other of said chambers, said plate having a relatively small area thereof exposed to ambient air to permit heat drainage from said chambers, and portions of opposed areas of said plate being shielded by said blocks from the air in said chambers and from ambient air.

CHARLES L. WATSON. 

